When a patient has a personality disorder and an eating disorder, the therapist would do well to first tackle the eating disorder. Personality disorders are intricate and intractable. They are rarely curable (though certain aspects, like obsessive-compulsive behaviours, or depression can be ameliorated with medication or modified). The treatment of personality disorders requires enormous, persistent and continuous investment of resources of every kind by everyone involved.
From the patient's point of view, the treatment of her personality disorder is not an efficient allocation of scarce mental resources. Neither are personality disorders the real threat. If one's personality disorder is cured but one's eating disorders are left untouched, one might die (though mentally healthy)...
An eating disorder is both a signal of distress ("I wish to die, I feel so bad, somebody help me") and a message: "I think I lost control. I am very afraid of losing control. I will control my food intake and discharge. This way I can control at least ONE aspect of my life."
This is where we can and should begin to help the patient - by letting her regain control of her life. The family or other supporting figures must think what they can do to make the patient feel that she is in control, that she is managing things her own way, that she is contributing, has her own schedules, her own agenda, and that she, her needs, preferences, and choices matter.
(From the book "Malignant Self-love: Narcissism Revisited" by Sam Vaknin - Click on this link to purchase: http://www.narcissistic-abuse.com/thebook.html)